[Read aloud] Fix translation runtime errors.

When languages are toggled on-off-on from language menu, devtools
shows an infinite loop of the below error. This slows down the
application and might be contributing to slower voice-downloading
notifications.

I'm not sure why, but the i18n call from the computed binding function
`computeAvailableLanguages_` was breaking things.

```
Uncaught
Error: Failed to execute `invoke` on 'CreateHTMLCallback': The provided callback is no longer runnable.
    at parseHtmlSubset (...)
    at LanguageMenuElement.i18n (...)
    at LanguageMenuElement.getNotificationText (...)
    ...
    at LanguageMenuElement._evaluateBinding (...)
    ...
```

Bug: 338042600
Change-Id: I45c945a6d29f0a993aa329f025ffe5cdcb7d94c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5515718
Reviewed-by: Kristi Saney <kristislee@google.com>
Reviewed-by: Lauren Winston <lwinston@google.com>
Commit-Queue: Eitan Goldberger <eitang@google.com>
Cr-Commit-Position: refs/heads/main@{#1296248}